1. Field of the Invention
The present invention relates to a base station control equipment initiatively performing channel control including soft hand-off in a radio communication system of a small zone scheme, a mobile station equipment used for providing a communication service in each wireless zone, and a radio communication system constituted by combining the base station control equipment and mobile station equipment.
2. Description of the Related Art
A CDMA system, which has confidentiality and interference resistibility as its basic nature and adapts to various channel allocations and multimedia communication, has recently been applied positively in a mobile communication system since technologies of transmit power control which can solve a near-far problem has been established.
Furthermore, in such a mobile communication system, mobile stations share a receiving/transmitting part interfacing with a radio transmission path without a great increase in a hardware scale so that they can establish and maintain individual paths in parallel with a plurality of Base Transceiver Stations (BTSs).
Consequently, the mobile stations accommodated in the mobile communication system of the CDMA system, when they locate around a border of a plurality of wireless zones, execute soft hand-off which is realized through effective use of the characteristics of the CDMA system, in cooperation with the BTSs to maintain high speech qualities of successful calls occurring in local stations.
FIG. 12 is a block diagram showing a configuration example of a mobile communication system to which the CDMA system is applied.
In FIG. 12, a base station controller 50 is connected to a not-shown exchange via a communication link 51 and is connected to BTSs 53-1 to 53-K via communication links 52-1 to 52-K respectively. A mobile station 70 locates in any of wireless zones 54-1 to 54-K formed by the BTSs 53-1 to 53-K respectively.
The base station controller 50 is composed of an exchange interfacing part 55 connected to the communication link 51, a transmission channel interfacing part 56 cascaded to this exchange interfacing part 55 and connected to one end of each of the communication links 52-1 to 52-K, and a processor (CPU) 57 connected to control terminals of these exchange interfacing part 55 and transmission channel interfacing part 56.
The mobile station 70 is composed of an antenna 71, a receiving/transmitting part 72 connected to a feeding point of this antenna 71, a microphone 73 and a receiver 74 connected to a modulation input and a demodulation output of this receiving/transmitting part 72 respectively, an operation/indication part 75, and a processor (CPU) 76 having input/output ports connected to this operation/indication part 75 and the receiving/transmitting part 72 respectively.
In the mobile communication system as configured above, when some call occurs in the mobile station 70, the processor 57 provided in the base station controller 50 transmits/receives predetermined information mutually to/from the exchange which call-processes this call via the exchange interfacing part 55 and the communication link 51, to perform channel control based on a prescribed procedure in cooperation with this exchange.
In a process of such channel control, the processor 57 transmits/receives predetermined control information via the transmission channel interfacing part 56 mutually to/from a single or a plurality of BTSs, out of the BTSs 53-1 to 53-K, which form a wireless zone where the mobile station 70 is located (supposed here to be the wireless zone 54-1 for simplification) and wireless zones adjoining the above wireless zone and which satisfy a predetermined condition, to take the initiative in processing for allotting radio channels to the aforesaid call (the mobile station 70). Note that these wireless zones are hereinafter referred to simply as ‘proper zones’ and a set of the BTSs forming the ‘proper zones’ are referred to as a ‘proper BTS’.
In the mobile station 70, the processor 76 transmits/receives the aforesaid control information mutually to/from the base station controller 50 via the receiving/transmitting part 72, the antenna 71, and the whole proper BTS.
The processor 76 also specifies a single or a plurality of radio channels given as the predetermined control information and allotted to each of the aforesaid ‘proper zones’ (hereinafter, referred to as a ‘proper channel’) when it identifies that the call occurring in a local station becomes a successful call based on the prescribed procedure of the channel control. Furthermore, the processor 76 instructs the receiving/transmitting part 72 to execute transmission/receipt to be executed via each of thus specified radio channels. The receiving/transmitting part 72 transmits an upstream speech signal given via the microphone 73 to the radio channels in parallel according to this instruction and combines downstream speech signals received via the radio channels in parallel to feed the downstream speech signals to the receiver 74.
In other words, in the mobile station 70, soft hand-off is executed smoothly with the receiving/transmitting part 72 being shared so that the speech quality of the speech signal of the successful call occurring in the local station as an originating call or a terminating call is highly maintained.
Incidentally, the processor 76 performs predetermined processing according to a request for origination, a response to the terminating call, and other requests and information which are given via the operation/indication part 75, and in the process of this processing, it outputs via the operation/indication part 75, for example, information indicating that the terminating call has occurred and information indicating a set number and others when necessary.
Meanwhile, in the process of selecting the aforesaid proper zones and allotting the proper channels, the processor 57 provided in the base station controller 50 obtains, via the transmission channel interfacing part 56, levels of reception waves arriving from the mobile station 70 at BTSs which are candidates for the proper zones (hereinafter referred to as ‘candidate proper BTS’), out of the BTSs 53-1 to 53-K, the levels being individually measured by these candidate proper BTSs.
The processor 57 then specifies the maximum level among thus obtained levels (hereinafter, a BTS giving the maximum level in this way is referred to as a ‘reference BTS’), and based on a known constitution including a gain of an antenna system of the reference BTS and the mobile station 70, it converts the maximum level to the relative distance r of the mobile station 70 to the reference BTS.
Furthermore, the processor 57 sets the number BSH of radio channels to be used for the soft hand-off of the mobile station 70 at a larger value as the relative distance r is longer.
Incidentally, as shown in FIG. 13, this number BSH is supposed to be set, for example, at the following values relative to a first to third threshold values r1, r2, and r3 which are determined in advance:
.'1'(r . r1);.'2'(r1 < r . r2); and.'3'(r2 < r . r3).
The processor 57 also selects BSH or less BTSs from the BTSs to which radio channels to be used effectively for a call can be allotted, and allots the proper channels individually to the selected BTSs.
In other words, the number of the proper channels allotted to any of mobile stations (including the mobile station 70) in order to realize the soft hand-off is set at a smaller value as the relative distance r to the nearest BTS is shorter, while it is set at a larger value as this relative distance r is longer.
Incidentally, a transmission characteristic of a radio transmission path formed between a mobile station and a BTS generally deteriorates to a greater extent as the aforesaid relative distance r is longer.
However, a larger number of the proper channels to be used for compensating for fluctuation of such a transmission characteristic are allotted to any of the mobile stations as the relative distance r to the nearest BTS is longer.
Furthermore, in the conventional example described above, it is avoided that a successful call precedingly occurring is preferentially allotted to available radio channels in spite of that the number of radio channels which each of the BTSs 53-1 to 53-K can allot is originally limited.
Therefore, a ‘hand-off successibility ratio’ is defined as the number of the proper channels which are allotted to each of the mobile stations to perform the soft hand-off. Properly selecting the mobile stations to perform the soft hand-off and properly setting the number BSH under the aforesaid processing can maintain high transmission quality of an uplink.
However, in the conventional example described above, the number of the radio channels through which each of the BTSs 53-1 to 53-K transmits in parallel becomes larger as the possibility that the mobile station is located in a position distant from the nearest base station is higher, which causes increase in a hardware scale and cost increase.
As for a downlink, the degree of interference with other radio channels (including radio channels allotted to the adjoining wireless zones) which are formed together with the aforesaid radio channels in a common frequency band increases, and consequently, the number of the radio channels (channel capacity) through which transmission with a desired transmission quality can be performed in parallel, sharing this frequency band, is not always maintained at a predetermined value.
Moreover, the number of the radio channels through which transmission can be performed in parallel in the above-mentioned frequency band is restricted, for example, to ‘64’ or smaller including transmission information of radio channels used for the channel control since a code division multiple access system employing a Walsh function with a code length ‘64’ is applied in a standardized IS-95.
In other words, the channel capacity (transmission quality) of the uplink shows a monotone increase relative to the aforesaid ‘hand-off successibility ratio’ as shown by the bold line in FIG. 14.
Furthermore, the channel capacity (transmission quality) of the downlink increases relative to the ‘hand-off successibility ratio’ in a region where the ‘hand-off successibility ratio’ is small as shown by the dotted line in FIG. 14.
On the other hand, in a region where the ‘hand-off successibility ratio’ exceeds a predetermined value (for example, ‘0.5’), the channel capacity (transmission quality) of the downlink decreases since the degree of the aforesaid interference increases.
Consequently, in the conventional example, when the number BSH is set, for example, at ‘3’ for all the mobile stations, the ‘channel capacity’ is a high value of 14.7 channel/cell for the uplink (FIG. 14(1)), while it is a small value of 13.0 channel/cell for the downlink (FIG. 14(2).
Incidentally, FIG. 14 shows the result of computer simulation executed under the following conditions:    .path loss exponent.=4;    .Standard Deviation of Shadowing.=8 dB;    .shadowing correlation.=0.5;    .the maximum soft hand-off branch number=3;    .application of a RAKE receiving system by a finger number ‘3’;    .required Eb/N0 of the downlink=5 dB;    .required Eb/N0 of the uplink=7 dB;    .a ratio of power allotment to radio channels except a speech channel of the downlink=0.2;    .not to execute voice activation (VOX) (a speech validity factor=1);    .to sectorize none of the wireless zones; and    .to completely execute the transmit power control.